Recently, electrophotographic image forming apparatuses forming images by melting toner images have been developed and broadly used as copiers, printers, multi-functional products, and the like.
Electrophotographic image forming apparatuses typically form images by passing a recording medium such as paper sheets bearing a toner image thereon through a fixing nip formed by a heated roller or belt to fix the toner image on the recording medium upon application of heat and pressure thereto.
Specifically, such image forming apparatuses form images, for example, by performing the following processes:    (1) charging a surface of a photoreceptor with a charger so that the photoreceptor has a charge thereon (charging process);    (2) irradiating the surface of the photoreceptor with laser light or LED (light emitting diode) light emitted by an irradiator according to image information to form an electrostatic latent image on the surface of the photoreceptor (irradiating process);    (3) developing the electrostatic latent image with a toner supplied from a developing device to form a visible image (toner image) on the surface of the photoreceptor (developing process);    (4) transferring the toner image onto a transfer belt (primary transfer process);    (5) re-transferring the toner image onto a recording medium (secondary transfer process);    (6) applying heat and pressure to the toner image using a fixing device to fix the toner image on the recording medium (fixing process).
The fixing device applies heat and pressure to the toner image on the recording medium using a combination of a roller and an endless belt while feeding the recording medium. Specifically, the fixing device typically includes a fixing member such as a roller or an endless belt, which heats the toner image, and a pressing member such as a roller or an endless belt, which is pressed to the fixing member. One example of the fixing device is a fixing device using a roller for each of the fixing member and the pressing member.
Since the nip of such a fixing device, which is formed by contacting a roller fixing member and a roller pressing member, cannot have a wide nip width, it becomes difficult to increase the heating time so as to be sufficient for melting a toner image when the printing speed is increased. In order to increase the nip width, there is a technique such that the thickness of the rubber layers on the rollers of such a fixing device is increased to increase the deformation amount of the rollers (rubber layers), resulting in increase of the nip width. However, such a fixing device has a drawback in that since the heat capacity of the rollers increases due to increase of the thickness of the rubber layers of the rollers, the rollers cannot be rapidly heated, thereby making it impossible to start a printing operation shortly after starting up the image forming apparatus (i.e., increase of start-up time).
In a fixing device having a roller fixing member and a belt pressing member, the nip width can be increased even when the rubber layer of the roller fixing member is relatively thin because the belt pressing member can be contacted with the surface of the roller fixing member over a relatively long distance, and therefore it become possible to increase the printing speed (fixing speed). In addition, it becomes possible to start a printing operation shortly after starting up the image forming apparatus (i.e., to decrease the start-up time) because the thickness of the rubber layer of the roller fixing member can be decreased, thereby decreasing the heat capacity of the roller fixing member.
However, such a fixing device has a drawback in that since the recording medium such as paper sheets is wound around the roller fixing member, the recording medium tends to be curled depending on the curvature of the roller fixing member.
In addition, when increasing the nip width in such a fixing device, the roller fixing member has to have a relatively large diameter. In this case, the heat capacity of the roller fixing member increases, thereby increasing the start-up time between start-up of the image forming apparatus and start of a printing operation.
In a fixing device having an endless belt fixing belt and an endless belt pressing member, it is possible to form a flat nip having a relatively long width, but at least one of the belts has to be rotated while tightly stretched by plural rollers. Therefore, the fixing device has a complex structure, and thereby the heat capacity of the fixing device is increased, resulting in increase of the start-up time of the image forming apparatus.
In attempting to solve such problems, a fixing device is proposed which has an endless belt, which is not tightly stretched; a heating member arranged in the endless belt to directly heat the endless belt; a pressing pad with a flat surface arranged in the endless belt; and a pressing roller having a soft rubber layer thereon and pressing the endless belt to form a fixing nip, wherein the endless belt is driven by the pressing roller so as to rotate.
When a toner image on a recording medium such as paper sheets is melted at a fixing nip of a fixing device, an adhesive force is generated, and the recording medium is attracted by the fixing member at the exit of the fixing nip. In this case, since the recording medium has large rigidity, the recording medium is generally peeled from the fixing member. However, when a thin recording medium having small rigidity is used and a solid image having a large area is formed thereon, a winding problem in that the recording medium is wound around the fixing member is caused.
In attempting to solve the winding problem, a fixing device is proposed in which a separating plate to separate a recording medium from a fixing member is arranged in the vicinity of the exit of the fixing nip. In addition, the fixing device includes a separating member (i.e., a guide), which is located inside the endless belt serving as the fixing member, to decrease the curvature radius of the endless belt so that the recording medium can be easily separated from the endless belt due to the rigidity thereof.
In addition, a fixing device is proposed in which a pressing pad arranged inside an endless belt has a convex portion on a downstream side from the fixing nip relative to the recording medium feeding direction to change the feeding direction of the endless belt toward the pressing roller at the exit of the fixing nip so that a recording medium can be easily separated from the endless belt.
In a fixing device such as the first-mentioned conventional fixing device, in which an endless belt is driven by a pressing roller, the resistance of the endless belt to feeding is preferably as small as possible to stably feed the endless belt. Therefore, in such a fixing device, the endless belt is arranged so as not to be contacted with members (such as guides arranged inside the endless belt and used for determining the feeding position of the endless belt) except for the pressing pad to form the fixing nip. However, in this fixing device, the endless belt is contacted with the guides at the end portions of the guides in the direction perpendicular to the feeding direction of the endless belt to prevent meandering of the endless belt. Therefore, the endless belt is slightly deformed at the end portions thereof because the end portions of the belt are contacted with such guides.
FIG. 4 is a side view illustrating a conventional fixing device used for image forming apparatuses. In the fixing device illustrated in FIG. 4, a pressing pad 33 and a heater 34 are arranged inside an endless belt 30. Positioning of the fed endless belt 30 is made by a guide member 32 serving as a first internal guide. Since a rotatable pressing roller 37 is pressed toward the pressing pad 33 with the endless belt 30 therebetween, a recording medium 13 such as films and papers and the endless belt 30 are fed by the pressing roller 37.
In this regard, when the recording medium 13 passes through a fixing nip 35, the endless belt 30 is deformed as illustrated in FIG. 5 due to the rigidity of the recording medium 13 and the adhesive force of the toner of a toner image on the recording medium 13.
FIG. 5 is a side view illustrating the conventional fixing device illustrated in FIG. 4 achieving a state, in which the endless belt 30 is deformed due to passing of the recording medium 13 bearing a toner image thereon through the fixing device.
Since the endless belt 30 is deformed at the exit of the fixing nip 35 as illustrated in FIG. 5, the rigidity (i.e., resistance to bending) of the recording medium 13 decreases in a range L, and therefore the recording medium 13 tends to be easily adhered to the endless belt 30, resulting in occurrence of the winding problem in that the recording medium is wound around the endless belt.
In this regard, even when the feeding direction of the endless belt 30 is changed toward the pressing roller 37 at the downstream side from the fixing nip 35 relative to the feeding direction of the recording medium 13, occurrence of this problem cannot be prevented because the endless belt 30 is deformed and the rigidity of the recording medium 13 decreases in the range L, thereby deteriorating the releasability of the recording medium 13 from the fixing member (endless belt 30).
For these reasons, the inventors recognized that there is a need for a fixing device which can securely feed a recording medium without causing the winding problem even when the recording medium has low rigidity and bears a solid image on the entire surface thereof.